HAMMER

tool designed for pounding or delivering repeated blows. Varied uses require a multiplicity of designs and weights. Hand hammers consist of a handle and striking head, with the head often made of metal with a hole in the centre to receive a wooden handle. Sometimes the entire hammer is forged or cast in one piece of metal. Surfaces of hammerheads vary in size, in angle of orientation to the handle (parallel or inclined), and in type of face (flat or convex). Carpenters' hammers often have a claw on the head for extracting nails. Weights range from a few ounces or grams up to 15 pounds (7 kg) for hammers used in breaking stones. Heavier hammers are power tools, among the largest being the pile driver. Trip-hammers depend on gravity for their impulse, but steam hammers often employ, in addition to gravity, a downward thrust from a steam-activated piston. Pneumatic hammers driven by air include the hammer drill, used on rock and concrete, in which the hammerhead is a drill head, and the riveting hammer, used principally in construction operations involving steel girders and plate.